This invention generally relates to apparatus and methods for winding a wire onto a structure, and more specifically, to apparatus and methods for winding a coated wire under tension around a peripheral wall of a structure to form a pretressed structure.
Prestressed structures often comprise a generally cylindrical concrete wall, with one or more layers of stressed wire wrapped about the cirumference of the wall; and these structures are used for many purposes, such as storage tanks for liquids and gases, silos, and pressure containment chambers. In the construction of such a structure, a steel wire is stressed and then coiled around a cylindrical concrete wall to exert a net compressive force therein, and the wire is then normally embedded in concrete or mortar. The force that the wire exerts on the concrete wall is usually large enough so that when the structure is filled with a fluid, there is always a net compressive force in the structural wall, even though the contents thereof tend to exert a tensive force in the wall.
In order to insure that the steel wire continues to exert the desired compressive force over the lifetime of the structure, it is important to avoid corrosion of the wire, which may be caused by moisture penetration, the source of which is either internal or external to the structure. Various techniques may be used to inhibit this corrosion; and, for example, the wire may be coated with a water resistant material such as polyvinyl chloride, a resin or an-epoxy.
While these coatings effectively inhibit corrosion, it is believed that they may not be completely successful at entirely preventing the corrosion. To elaborate, it has been found that tiny flaws and defects, such as cracks, tears, or other openings, may form in the wire coating during the wire coating process or as the wire is stressed prior to being wrapped around the structure. If these flaws are not fixed, water or other substances may leak inward through the flaws and eventually have a corrosive effect on the steel wire core.